Diet analysis

Mediterranean shags are fisheating birds

Mediterranean shags are foraging exclusively for fish but not for any species. The size of fish and their spatial niche are two important factors that define them as prey of the shags. They mostly prey on poorly mobile bottom dwelling fish, but occasionally also on schools of pelagic fish, especially when these occur in shallow coastal waters. The shags usually forage solitarily or in small groups, but cases of large groups of shags preying on fish flock are not such a rare occasion. Often they are attended by other species of marine birds, like gulls (Larus sp.).

Availability of prey defines distribution of seabirds

Distribution of seabirds at sea is often driven by availability of their prey, especially in the non-breeding season when birds are not bound to their nesting sites. Studying of their diet and foraging habits therefore significantly contributes to the identification of the most important areas for their survival, provided that data on spatial distribution and seasonal availability of their prey are available, too.

The Mediterranean shags attended with gulls socially foraging for fish

Pellets of shags contain important information

Studying of avian diet is based on different methods. A widely used method is an analysis of indigested particles of food, which is appropriate for studying diet of the Mediterranean shag as well. The shags consume whole fish and eliminate the indigested particles in mucous pellets. The pellets stick on the ground (buoys, rocks, platforms) and quickly become dry. The indigested particles include scales, teeth and otholits, representing material for analysis of species composition and size of prey.

Diet analysis

In the project SIMARINE-NATURA we will analyse at least 500 pellets of Mediterranean shags, which will be collected mostly at communal roosting sites in summer and early autumn. In winter and spring the pellets are hardly available in the project area due to very small numbers of shags present and regular rain which partly or wholly washes the pellets from the ground. The results of the analysis will contain species composition of the prey, relative abundance of fish species in the pellets and size of prey.